This invention relates to an electric motor drive system which estimates the variation of the rotational speed of an electric motor and controls the variation to render it to a predetermined value such as zero, and more particularly to a technique for performing such control without using any speed sensor.
The twin drive system, which is an apparatus for driving two electric motors whose rotating shafts are physically and mechanically coupled to each other via a gear box so as to rotationally drive a common load, has found its broad application in the manufacturing process for various industrial materials like rubber and cement.
In such a twin drive system, the two electric motors are sometimes driven by two respective power converters so that the speeds of the two motors can be variably controlled. When such two electric motors coupled to a common mechanical load are driven, the rotational speeds of the two motors must be adjusted to the same value by the help of a common mechanical system.
However, if the cogged wheels in the common mechanical system are not uniformly worked, there sometimes arises unbalance on the mechanical torques exerted on the rotary shafts of the electric motors so that the rotational speeds of the motors are rendered uneven with each other. The uneven rotational speeds may impose physically adverse effect on the mechanical load and the common mechanical system (such as gear clattering).
To eliminate such adverse effect, the rotational speeds of the motors must be adjusted to the same value, even if there are unbalanced mechanical loads imposed on the rotary shafts of the two electric motors. Many examples of the conventional methods for speed control of electric motors have resorted to the use of speed sensors. However, if the ambient temperature around the electric motors is very high, or if the motors are subjected to vibratory conditions, then the installation of speed sensors becomes difficult. In such a case, it is preferable not to use speed sensors.
For example, JP-A-6-346157 discloses a conventional method for controlling the twin drive system without using speed sensors. According to the disclosed method, the difference between the torque currents of the respective electric motors is used to control the frequency so that the torque balance between the two motors can be controlled.